U.S. patent number 4,157,495 [Application Number 05/842,368] was granted by the patent office on 1979-06-05 for nuclear magnetic resonance gyro.
This patent grant is currently assigned to Litton Systems, Inc.. Invention is credited to Bruce C. Grover, Edward Kanegsberg, John G. Mark, Roger L. Meyer.
United States Patent |
4,157,495 |
Grover , et al. |
June 5, 1979 |
**Please see images for:
( Certificate of Correction ) ** |
Nuclear magnetic resonance gyro
Abstract
A nuclear magnetic resonance gyroscope is disclosed that
operates on the principle of sensing angular rotation rate as a
shift in the Larmor frequency of one or more nuclear species that
possess nuclear magnetic moments.
Inventors: |
Grover; Bruce C. (Thousand
Oaks, CA), Kanegsberg; Edward (Pacific Palisades, CA),
Mark; John G. (Pasadena, CA), Meyer; Roger L. (Canoga
Park, CA) |
Assignee: |
Litton Systems, Inc. (Woodland
Hills, CA)
|
Family
ID: |
24872234 |
Appl.
No.: |
05/842,368 |
Filed: |
October 14, 1977 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
714978 |
Aug 14, 1976 |
|
|
|
|
Current U.S.
Class: |
324/302 |
Current CPC
Class: |
G01R
33/26 (20130101); G01C 19/62 (20130101) |
Current International
Class: |
G01C
19/58 (20060101); G01C 19/62 (20060101); G01R
33/24 (20060101); G01R 33/26 (20060101); G01R
033/08 () |
Field of
Search: |
;324/.5R,.5F,.5A
;331/94.5H |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tokar; M.
Attorney, Agent or Firm: Gillmann; Harold E. Brown; Ernest
L.
Parent Case Text
CROSS-REFERENCE TO A RELATED APPLICATION
This is a continuation of Application Ser. No. 714,978 filed Aug.
14, 1976, now abandoned.
Claims
We claim:
1. A nuclear magnetic resonance detection device comprising:
a nuclear magnetic resonance cell;
a gas vapor of an optically pumpable substance that possesses a
magnetic moment and is capable of being optically pumped, said
optically pumpable substance being contained in said cell;
at least one nuclear moment gas each having a nuclear magnetic
moment also contained in said cell, the nuclear magnetic moments of
each said nuclear moment gas being at least partially aligned;
means for applying a steady magnetic field to said cell;
first means for illuminating said cell with optical pumping light
capable of partially aligning the magnetic moments of said
optically pumpable substance in one direction by absorption of said
light;
means for precessing said aligned nuclear magnetic moments of each
said nuclear moment gas about the direction of the steady magnetic
field at the respective Larmor precession frequencies of each said
gas;
means for applying an AC carrier magnetic field to the cell;
second means of illuminating said cell with detection light of a
wavelength approximately equal to a wavelength which can be
absorbed by the optically pumpable substance;
means for applying said detection light with a directional
component orthogonal to the direction of said AC carrier magnetic
field to produce modulations in the intensity of the transmitted
part of said detection light substantially at the frequency of at
least one harmonic, including the fundamental of said AC carrier
magnetic field thereof;
means for detecting at least one of said modulations in the
intensity of the transmitted part of said detection light; and
means for electrically demodulating said detected light intensity
modulations to obtain a signal varying at the Larmor precession
frequency of each said nuclear moment gas and with amplitude
proportional to the degree of alignment of said nuclear magnetic
moments of each said gas.
2. The device as claimed in claim 1 wherein said optically pumpable
substance is an alkali metal.
3. The device as claimed in claim 1 wherein each said nuclear
moment gas is a noble gas.
4. The device as claimed in claim 1 wherein said steady magnetic
field has a component parallel to the direction of said optical
pumping light.
5. The device as claimed in claim 4 wherein said nuclear magnetic
moments of each nuclear moment gas are partially aligned by
collisions of atoms of each said nuclear moment gas with atoms of
said optically pumpable substance to partially transfer said
alignment from said substance to each said gas.
6. The device as claimed in claim 1 wherein said light intensity
modulations of said transmitted part of said detection light are
produced by absorption of said detection light by the optically
pumpable substance.
7. The device as claimed in claim 1 further including means for
accurately measuring the magnitude and direction of said steady
magnetic field.
8. A nuclear magnetic resonance device as defined in claim 1
further including means for accurately controlling the magnitude
and direction of said steady state magnetic field.
9. A nuclear magnetic resonance oscillator comprising the device as
claimed in claim 1 wherein said precessing means comprises means
for applying an AC feedback magnetic field at said detected Larmor
precession frequency of each said nuclear moment gas in a direction
orthogonal to the direction of said steady magnetic field, further
including means for detecting the phase of said Larmor precession
frequency, and wherein each said detected phase of said Larmour
precession frequency is utilized to control the respective phase of
said AC feedback magnetic field substantially in quadrature with
the phase of said processing nuclear magnetic moments of each said
gas thereby causing a sustained precession of said moments of each
said gas.
10. A nuclear magnetic resonance oscillator as claimed in claim 9
further including means for accurately measuring the magnitude and
direction of said steady magnetic field.
11. A nuclear magnetic resonance oscillator as claimed in claim 9
further including means for accurately controlling the magnitude
and direction of said steady magnetic field.
12. A nuclear magnetic resonance gyroscope comprising a nuclear
magnetic resonance oscillator as claimed in claim 9 further
including means for sensing angular displacements of said gyroscope
about the direction of said steady magnetic field as changes in
phase of the Larmor procession frequency of at least one of said
nuclear moment gases.
13. A nuclear magnetic resonance detection device as defined in
claim 1 wherein the steady magnetic field has a particular
magnitude that causes the precession of the magnetic moments of
said optically pumpable substance to occur substantially at a
harmonic of the frequency of said applied AC carrier magnetic
field, including the fundamental frequency, and wherein the
direction of said steady magnetic field is substantially parallel
to the direction of said applied AC carrier magnetic field.
14. A nuclear magnetic resonance detection device as claimed in
claim 13 wherein said steady magnetic field applied to said nuclear
magnetic resonance cell exceeds 0.01 gauss.
15. A nuclear magnetic resonance detection device as claimed in
claim 13 wherein said detection light is of substantially the same
wavelength as said optical pumping light.
16. A nuclear magnetic resonance detection device as claimed in
claim 15 wherein said detection light and said optical pumping
light originates from the same light source.
17. A nuclear magnetic resonance detection device as claimed in
claim 16 wherein said detection light and said optical pump light
are comprised of parallel components of a single beam of light from
said light source.
18. A nuclear magnetic resonance detection device claimed in claim
16 wherein said detection light and said optical pumping light are
comprised of nonparallel components of a single beam of light from
said light source.
19. A nuclear magnetic resonance oscillator comprising the
detection device as claimed in claim 13 wherein said means for
causing the aligned nuclear magnetic moments of each said nuclear
moment gas to precess about the direction of said steady magnetic
field comprises means for applying an AC feedback magnetic field in
a direction orthogonal to the direction of said steady magnetic
field at the said detected Larmor precession frequency of each said
nuclear moment gas, further including means for detecting the phase
of each said Larmor precession frequency and wherein said detected
phase of each said Larmor precession frequency is utilized to
control the phase of the corresponding AC feedback magnetic field
to be essentially in quadrature with the respective phase of said
precessing nuclear magnetic moments of said gas thereby causing a
sustained precession of said moments of said gas.
20. A nuclear magnetic resonance oscillator as defined in claim 19
further including means for electrically demodulating said detected
light intensity modulations to obtain control signals having
amplitudes proportional to magnetic field components transverse to
said AC carrier magnetic field, and further including means for
measuring or controlling said transverse field components.
21. A nuclear magnetic resonance gyroscope comprising the
oscillator claimed in claim 19 further including means for sensing
angular displacements or angular rates of the device about the
direction of said steady magnetic field as changes in phase or as
changes in frequency, respectively, of the Larmor precession
frequencies of at least one said nuclear moment gas.
22. A nuclear magnetic moment alignment device comprising:
a nuclear magnetic resonance cell;
a gas or vapor of an optically pumpable substance that possesses a
magnetic moment and is capable of being optically pumped, said
optically pumpable substance being contained in said cell;
first and second noble gases, chosen from the class consisting of
xenon isotopes having a nuclear magnetic moment and krypton
isotopes having a nuclear magnetic moment, and contained in said
cell;
means for applying a steady state magnetic field to said cell;
and
means for illuminating said cell with optical pumping light capable
of partially aligning in one direction by absorption of said light
the magnetic moments of said optically pumpable substance to cause
the nuclear magnetic moments of said first and second noble gases
to be partially aligned by collisions of atoms of said optically
pumpable substance with atoms of said noble gases.
23. A nuclear magnetic moment alignment device as claimed in claim
22 wherein the said steady magnetic field has a component parallel
to the direction of said optical pumping light.
24. A nuclear magnetic moment alignment device as claimed in claim
23, wherein a substantial amount of at least one buffer gas is also
contained in said cell.
25. A nuclear magnetic moment alignment device as claimed in claim
24 wherein said buffer gas is helium.
26. A nuclear magnetic moment alignment device as claimed in claim
24 wherein said buffer gas is nitrogen.
27. A nuclear magnetic moment alignment device as claimed in claim
22 wherein said optically pumpable substance is an alkali
metal.
28. A nuclear magnetic alignment device as claimed in claim 27
wherein said alkali metal is rubidium.
29. A nuclear magnetic moment alignment device comprising:
a nuclear magnetic resonance cell;
a gas or vapor of an optically pumpable substance that possesses a
magnetic moment and is capable of being optically pumped, said
optically pumpable substance being contained in said cell;
xenon-129 and krypton-83 gases each having a nuclear magnetic
moment and contained in said cell;
means for applying a steady state magnetic field to said cell;
and
means for illuminating said cell with optical pumping light capable
of partially aligning in one direction, by absorption of said
light, the magnetic moments of said optically pumpable substance to
cause the nuclear magnetic moments of said xenon-129 and krypton-83
gases to be partially aligned by collisions of atoms of said
optically pumpable substance with atoms of said xenon129 and
krypton-83.
30. A nuclear magnetic moment alignment device as claimed in claim
22 further including means for accurately measuring and controlling
the magnitude and direction of steady magnetic field.
31. A nuclear magnetic resonance detection device comprising the
alignment device as claimed in claim 20 further including means for
precessing the aligned nuclear magnetic moments of said two noble
gases about the direction of the steady magnetic field at the
respective Larmor precession frequencies of said two noble gases
and means for detecting said Larmor precession frequencies.
32. A nuclear magnetic resonance detection device as claimed in
claim 31 further including means for utilizing the difference
between the two Larmor precession freguencies of said two noble
gases for accurately fixing the magnitude of the component of said
steady magnetic field which is parallel to said AC carrier magnetic
field to a predetermined level.
33. A nuclear magnetic resonance oscillator comprising the
detection device as claimed in claim 31 wherein said means for
causing the aligned nuclear moments of said two noble gases to
precess about the direction of said steady magnetic field comprises
means for applying in a direction orthogonal to the direction of
said steady magnetic field two AC feedback magnetic fields at said
respective detected Larmor precession frequencies of said two noble
gases, further including means for detecting the phases of said
Larmor precession frequencies, and wherein said detected phases of
said Larmor precession frequencies are utilized to control the
respective phases of said AC feedback magnetic fields to be
essentially in guadrature with the respective phases of said
precessing nuclear magnetic moments of said two noble gases thereby
causing sustained precession of said moments of said two noble
gases.
34. A nuclear magnetic resonance gyroscope comprising the
oscillator as claimed in claim 33 further including means for
sensing angular displacements or angular rates of the device about
the direction of said steady magnetic field as changes in phase or
as changes in frequency, respectively, of the Larmor precession
frequencies of at least one of the said two noble gases.
35. A nuclear magnetic resonance detection device as claimed in
claim 31 further including means for applying an AC carrier
magnetic field to said nuclear magnetic resonance cell, and means
for illuminating said cell with detection light of a wavelength
approximately equal to a wavelength which can be absorbed by the
optically pumpable substance, wherein said detection light has a
directional component orthogonal to said AC carrier magnetic field,
further including means for producing and detecting modulation in
the intensity of said detection light at approximately the
frequency of at least one harmonic of said AC carrier magnetic
field and wherein said means for detecting said Larmor precession
frequencies of said two noble gases comprises additional means for
electrically demodulating at least one said detected light
intensity modulation to obtain a signal varying at the Larmor
precession frequencies of said noble gases and with amplitudes
proportional to the degree of alignment of said nuclear magnetic
moments of said gases.
36. A nuclear magnetic resonance detection device as claimed in
claim 34 further including means for accurately measuring and
controlling the magnitude and direction of said steady magnetic
field.
37. A nuclear magnetic resonance detection device as claimed in
claim 35 wherein the steady magnetic field has a particular
magnitude that causes the precession of the magnetic moments of
said optically pumpable substance to occur substantially at a
harmonic of the frequency of said applied AC carrier magnetic
field, including the fundamental, and wherein the direction of said
steady magnetic field is substantially parallel to the direction of
said applied AC carrier magnetic field.
38. A nuclear magnetic resonance detection device as defined in
claim 37 further including means for accurately measuring or
controlling the magnitude and direction of said steady magnetic
field.
39. A nuclear magnetic resonance detection device as claimed in
claim 38 wherein said means for controlling said steady magnetic
field comprises means for electrically demodulating said detected
light intensity modulations to obtain control signals having
amplitudes proportional to magnetic field components transverse to
said AC carrier magnetic field, and further including means for
measuring and controlling said transverse field components and
means for comparing the difference frequency between the two Larmor
precession frequencies to a precision difference frequency
reference to adjust to a predetermined level the magnitude of the
component of said steady magnetic field which is parallel to the
direction of said AC carrier magnetic field.
40. A nuclear magnetic resonance gyroscope comprising the detection
device as claimed in claim 39 further including means for sensing
angular displacements or angular rates of the device about the
direction of said steady magnetic field as changes in phase or as
changes in frequency, respectively, of the Larmor precession
frequencies of at least one of the said two noble gases.
41. A nuclear magnetic resonance gyroscope as claimed in claim 39
further including means for comparing the change in said Larmor
precession frequency of one of the noble gases to a precision
Larmor frequency reference and means for deriving the frequency of
said AC carrier magnetic field and the frequency of said precision
difference frequency reference and the frequency of said precision
Larmor frequency reference from a single precision frequency
source.
42. A nuclear magnetic moment alignment device comprising:
a nuclear magnetic resonance cell;
a gas or vapor of an optically pumpable substance that possesses a
magnetic moment and is capable of being optically pumped, said
optically pumpable substance being contained in said cell;
at least one nuclear moment gas having a nuclear magnetic moment
and chosen from the class consisting of isotopes of xenon and
krypton, and contained in said cell;
a substantial amount of at least one buffer gas contained in said
cell;
means for applying a steady magnetic field to said cell; and
means for illuminating said cell with optical pumping light capable
of partially aligning in one direction, by absorption of said
light, the magnetic moments of said optically pumpable substance to
cause the nuclear magnetic moments of each said nuclear moment gas
to be partially aligned by collisions of atoms of said optically
pumpable substance with atoms of each said nuclear moment gas.
43. A nuclear magnetic moment alignment device comprising:
a nuclear magnetic resonance cell;
a gas or vapor of an optically pumpable substance that possesses a
magnetic moment and is capable of being optically pumped, said
optically pumpable substance being contained in said cell;
at least xenon-129 having a nuclear magnetic moment and contained
in said cell;
a substantial amount of at least one buffer gas contained in said
cell;
means for applying a steady magnetic field to said cell; and
means for illuminating said cell with optical pumping light capable
of partially aligning in one direction, by absorption of said
light, the magnetic moments of said optically pumpable substance to
cause the nuclear magnetic moments of said xenon-129 to be
partially aligned by collisions of atoms of said optically pumpable
substance with atoms of said xenon-129.
44. A nuclear magnetic moment alignment device as claimed in claim
42 wherein said optically pumpable substance is an alkali
metal.
45. A nuclear magnetic moment alignment device as claimed in claim
44 wherein said alkali metal is rubidium.
46. A nuclear magnetic moment alignment device as claimed in claim
42 wherein said buffer gas is helium.
47. A nuclear magnetic moment alignment device as claimed in claim
42 wherein said buffer gas is nitrogen.
48. A nuclear magnetic moment alignment device as defined in claim
42 wherein further means are provided for accurately measuring or
controlling the magnitude and direction of said steady magnetic
field.
49. A nuclear magnetic resonance detection device comprising an
alignment device as claimed in claim 42 further including means for
precessing the nuclear magnetic moments of each said nuclear moment
gas about the direction of the steady magnetic field at the
respective Larmor precession frequency of said gas and means for
detecting said Larmor frequency.
50. A nuclear magnetic resonance oscillator comprising:
a nuclear magnetic resonance cell;
a gas or vapor of an optically pumpable substance that possesses a
magnetic moment and is capable of being optically pumped, said
optically pumpable substance being contained in said cell;
a substantial amount of at least one buffer gas contained in said
cell;
means for applying a steady magnetic field to said cell;
means for illuminating said cell with optical pumping light capable
of partially aligning in one direction, by absorption of said
light, the magnetic moments of said optically pumpable substance to
cause the nuclear magnetic moments of each said nuclear moment gas
to be partially aligned by collisions of atoms of said optically
pumpable substance with atoms of each said nuclear moment gas;
means for causing the aligned nuclear magnetic moments of each said
nuclear moment gas to precess about the direction of said steady
magnetic field at the respective Larmor precession frequency of
said gas includes means for detecting said Larmor frequency and
means for applying in a direction orthogonal to the direction of
said steady magnetic field an AC feedback magnetic field at the
said detected Larmor precession frequency of each said nuclear
moment gas, and further including means for detecting the phase of
said Larmor precession frequency to control the phase of said AC
feedback magnetic field to be essentially in quadrature with the
respective phase of said precessing nuclear magnetic moments of
said gas to cause a sustained precession of said moments of each of
said gases.
51. A nuclear magnetic resonance gyroscope comprising an oscillator
as claimed in claim 50 further including means for sensing angular
displacements or angular rates of the device about the direction of
said steady magnetic field as changes in phase or as changes in
frequency, respectively, of the Larmor precession frequencies of at
least one of the said nuclear moment gases.
52. A nuclear magnetic resonance detection device comprising:
a nuclear magnetic resonance cell;
a gas or vapor of an optically pumpable substance comprising a
substance that possesses a magnetic moment and is capable of being
optically pumped, said optically pumpable substance being contained
in said cell;
at least one nuclear moment gas having a nuclear magnetic moment
and contained in said cell;
a substantial amount of at least one buffer gas contained in said
cell;
means for applying a steady magnetic field to said cell;
means for illuminating said cell with optical pumping light capable
of partially aligning in one direction, by absorption of said
light, the magnetic moments of said optically pumpable substance to
cause the nuclear magnetic moments of each said nuclear moment gas
to be partially aligned by collisions of atoms of said optically
pumpable substance with atoms of each said nuclear moment gas;
means for precessing the nuclear magnetic moments of each said
nuclear moment gas about the direction of the steady magnetic field
at the respective Larmor precession frequency of said gas;
means for detecting said Larmor frequency;
means for applying an AC carrier magnetic field to said cell;
means for illuminating said cell with detection light of a
wavelength approximately equal to a wavelength which can be
absorbed by the optically pumpable substance, wherein said
detection light has a directional component orthogonal to said AC
carrier magnetic field;
means for producing and detecting modulations in the intensity of
said detection light at or near the frequency of at least one
harmonic of said AC carrier magnetic field frequency;
said means for detecting said Larmor precession frequency of each
said nuclear moment gas comprising additional means for
electrically demodulating at least one of said detected light
intensity modulations to obtain a signal varying at the Larmor
precession frequency of each said nuclear moment gas and with
amplitudes proportional to the degree of alignment of said nuclear
magnetic moments of each said gas.
53. A nuclear magnetic resonance detection device comprising:
a nuclear magnetic resonance cell;
a gas or vapor of an optically pumpable substance comprising a
substance that possesses a magnetic moment and is capable of being
optically pumped, said optically pumpable substance being contained
in said cell;
at least one nuclear moment gas each having a nuclear magnetic
moment also contained in said cell, the nuclear magnetic moments of
each said nuclear moment gas being at least partially aligned;
means for applying a steady magnetic field to said cell;
first means for illuminating said cell with optical pumping light
capable of partially aligning in one direction by absorption of
said light, the magnetic moments of said optically pumpable
substance;
means for precessing said aligned nuclear magnetic moments of each
said nuclear moment gas about the direction of the steady magnetic
field at the respective Larmor precession frequencies of each said
gas;
means for applying an AC carrier magnetic field to the cell;
second means for illuminating said cell with detection light of a
wavelength equal to or approximately equal to a wavelength which
can be absorbed by the optically pumpable substance;
said first and second means illuminating said cell with separate
beams of light impinging from different directions;
means for applying said detection light with a directional
component orthogonal to the direction of said AC carrier magnetic
field to produce modulations in the intensity of at least one
harmonic, including the fundamental of the transmitted part of said
detection light substantially at frequency of said AC carrier
magnetic field thereof;
means for detecting at least one of said m modulations in the
intensity of the transmitted part of said detection light; and
means for electrically demodulating said detected light intensity
modulations to obtain a signal varying at the Larmor precession
frequency of each said nuclear moment gas and with amplitude
proportional to the degree of alignment of said nuclear magnetic
moments of each said gas.
54. A nuclear magnetic resonance unit as claimed in claim 52
wherein said first means for illuminating said cell with optical
pumping light illuminates said cell with light along a direction
substantially parallel to said steady magnetic field and said
second means for illuminating said cell with detection light
illuminates said cell with light along the direction substantially
orthogonal to said steady magnetic field.
55. A nuclear magnetic resonance gyro for producing signals
representative of the angular displacement of said gyro about a
sensing axis comprising:
a nuclear magnetic resonance cell;
a gas or vapor of an optically pumpable substance that possesses a
magnetic moment and is capable of being optically pumped, said
optically pumpable substance being contained in said cell;
two nuclear moment gases, each having a nuclear magnetic moment,
contained in said cell, the nuclear magnetic moments of each said
nuclear moment gas being at least partially aligned by collisions
of atoms of each said nuclear magnetic moment gas with atoms of
said optically pumpable substance to partially transfer said
alignment from said substance to each said gas;
means for applying a steady magnetic field to said cell
substantially in the direction of a predetermined sensing axis,
designated the z axis;
means for illuminating said cell with optical pumping light capable
of partially aligning the magnetic moments of said optically
pumpable substance in said z direction by absorption of said light,
said light having at least a component in the direction of said z
axis;
means for applying an AC carrier magnetic field, in the direction
of said z axis, to said cell;
means for illuminating said cell with detection light of a
wavelength approximately equal to a wavelength which can be
absorbed by said optically pumpable substance, said detection light
having at least a component perpendicular to said z axis to produce
modulations in the intensity of the transmitted part of said
detection light substantially at the frequency of at least one
harmonic, including the fundamental, of the frequency of said AC
carrier magnetic field;
means for precessing said aligned nuclear magnetic moments of each
said nuclear moment gas about said z axis at the respective Larmor
precession frequencies of each said gas, including means for
applying an AC feedback magnetic field at said detected Larmor
precession frequencies of each said nuclear moment gas in a
direction orthogonal to said z axis and further including means for
detecting the phase of said Larmor precession frequencies, said
detected phase of said Larmor precession frequencies being used to
control the respective phases of said AC feedback magnetic fields
substantially in quadrature with the phase of said precessing
nuclear magnetic moments of each said gas to cause a sustained
precession of said moments of each said gas;
means for detecting modulations in the intensity of the transmitted
part of said detection light and for changing said detected
modulations into electrical signals;
means for electrically demodulating said modulation signals to
obtain signals varying at the Larmor precession frequencies of each
of said nuclear moment gases, and a signal varying at the
difference between said Larmor precession frequencies, with
amplitudes of said signals proportional to the degree of alignment
of said nuclear magnetic moments of each said gas;
means for accurately controlling the magnitude and direction of
said steady state magnetic field; and
means for producing signals which are a measure of angular
displacement of said gyroscope about said z axis as changes in
phase of the Larmor precession frequency of at least one of said
nuclear moment gases.
56. A gyroscope as recited in claim 55 wherein said steady magnetic
field has a particular intensity to cause the precession of the
magnetic moments of said optically pumpable substance to occur
substantially at a harmonic of the frequency of said applied AC
carrier magnetic field, including the fundamental frequency
thereof.
57. Apparatus recited in claim 56 in which said optically pumpable
substance is an alkali metal.
58. The apparatus as recited in claim 56 wherein each said nuclear
moment gas is chosen from the class consisting of isotopes of xenon
having a nuclear magnetic moment and isotopes of krypton having a
nuclear magnetic moment.
59. The apparatus as recited in claim 58 wherein said nuclear
moment gases are xenon-129 and krypton-83.
60. The apparatus as recited in claim 56 wherein said detection
light is substantially the same wavelength as said optical pumping
light.
61. The apparatus as recited in claim 60 wherein said detection
light and said optical pumping light originates from the same light
source.
62. The apparatus as recited in claim 61 wherein said detection
light and said optical pumping light are parallel components of a
single beam of light from said light source.
63. The apparatus as recited in claim 61 wherein said detection
light and said optical pumping light are not parallel components of
a single beam of light from said light source.
64. Apparatus as recited in claim 60 wherein J.sub.0
(.gamma.H.sub.1 /.omega..sub.c)=J.sub.2 (.gamma.H.sub.1
/.omega..sub.c), and J.sub.0 is a Bessel function of the first kind
of order zero, J.sub.2 is a Bessel function of the first kind of
order two, .gamma. is the gyromagnetic ratio of said pumpable
substance, H.sub.1 is the AC component of the magnetic field in the
direction of the z axis, and .omega..sub.c is the angular frequency
of the AC carrier.
65. Apparatus as recited in claim 64 wherein said demodulation
means comprises a carrier signal detector receiving signals from
said photo detector, signals at said carrier frequency, and signals
at twice said carrier frequency to produce a nuclear precession
signal and x and y axes magnetic control signals, where the x and y
axes are mutually orthogonal axes perpendicular to said z axis, and
further comprising a nuclear precession light separator for
producing signals at the Larmor precession frequencies of said
gases and a signal at the difference frequency between the Larmor
precession frequencies of said gases to produce a control signal
for said AC magnetic field on said x axis and for controlling
increments in the z axis magnetic field.
66. Apparatus as recited in claim 65 and further comprising:
means for generating a signal at the predicted Larmor frequency of
one of said gases; and frequency comparator means for receiving
signals from said frequency means and said nuclear precession
signal separator to produce a signal at a phase which is at the
difference phase between said received signals, whereby said
last-produced signal is a measure of the angle said gyro has turned
about said z axis.
Description
BACKGROUND OF THE INVENTION
This invention relates to the creation and detection of nuclear
magnetic resonance. More particularly this invention relates to the
application of nuclear magnetic resonance to a gyroscope.
A number of approaches have been suggested in the prior art for
implementing the basic concept of a nuclear magnetic resonance NMR
gyroscope. In general, they utilize a nuclear magnetic resonance
controlled oscillator and derive rotational information from the
phases of the nuclear moment Larmor precession signals by means of
suitable phase comparison and magnetic field control circuitry.
In general, these devices contain significant deficiencies which
limit the development of a useful instrument. For instance, such
devices have been limited by relatively short relaxation times of
the gases which have been employed. Also, the strong direct
coupling between these gases and the light which is employed as the
means of magnetic moment alignment or magnetic moment detection can
limit both the relaxation times and the signal-to-noise ratio, and
therefore can also limit the potential usefulness of such
instruments.
SUMMARY OF THE INVENTION
A nuclear magnetic resonance (hereinafter referred to as "NMR")
gyroscope is disclosed that operates on the principle of sensing
inertial angular rotation rate or angular displacement about a
sensitive axis of the device as a shift in the Larmor precession
frequency or phase, respectively, of one or more isotopes that
possess nuclear magnetic moments. The gyroscope is composed of an
angular rotation sensor and associated electronics. The principal
elements of the sensor are a light source, an NMR cell, a
photodetector, a set of magnetic shields and a set of magnetic
field coils. The principal elements of the electronics are signal
processing circuits for extracting the Larmor precession frequency
and phase information as well as circuits for generating and
controlling various magnetic fields, both steady and varying
sinusoidally with time, that are necessary for the proper operation
of the device.
The NMR cell is mounted within a set of magnetic shields in order
to attenuate external magnetic fields to acceptably low levels.
Magnetic field coils are used to apply very uniform magnetic fields
to the NMR cell. Both a steady field and an AC carrier field are
applied along the sensitive axis of the device and AC feedback
fields are applied along one of the transverse axes. The DC
magnetic fields along both transverse axes are controlled to be
virtually zero. The NMR cell contains an alkali metal vapor, such
as rubidium, together with two isotopes of one or more noble gases,
such as krypton-83 and xenon-129. A buffer gas such as helium may
also be contained in the cell.
The NMR cell is illuminated by a beam of circularly polarized light
that originates from a source such as a rubidium lamp and which
passes through the cell at an angle with respect to the steady
magnetic field. Absorption of some of this light causes the atomic
magnetic moments of the rubidium atoms to be partially aligned in
the direction of the steady magnetic field. This alignment is
partially transferred to the nuclear magnetic moments of the noble
gases and these moments are caused to precess about the direction
of the steady magnetic field, which in turn creates magnetic fields
that rotate at the respective Larmor precession frequencies of the
two noble gases. These rotating fields modulate the precessional
motions of the rubidium magnetic moments, which in turn produces
corresponding modulations of the transmitted light, thereby making
it possible to optically detect the Larmor precession frequencies
of the two noble gases.
The modulations of the light intensity are converted into
electrical signals by a photodetector and these signals are then
electronically demodulated and filtered to provide signals at the
Larmor precession frequencies of the two noble gases. The
difference between the two precession frequencies is used to
accurately control the steady magnetic field so that it is
constant. One of the noble gas precession frequencies is compared
to a precision reference frequency and the resulting difference
frequency is the angular rotation rate of the gyroscope.
The two detected noble gas precession signals are also used to
generate two AC feedback magnetic fields at the Larmor precession
frequencies of the noble gases, and these are responsible for
sustaining the precession of the nuclear magnetic moments of the
noble gases. The use of an AC carrier magnetic field facilitates
the optical detection of the precessing noble gas moments as well
as providing means for controlling the DC magnetic fields along the
two transverse axes of the gyroscope.
According to the invention, the NMR gyroscope includes the means
for the simultaneous alignment of the nuclear magnetic moments of
at least two nuclear moment gases, thereby constituting a nuclear
magnetic moment alignment device; the means for achieving sustained
precession of these moments, thereby constituting a nuclear
magnetic resonance oscillator capable of sustained oscillations;
the means for the optical detection of these precessing nuclear
moments, thereby constituting a nuclear magnetic resonance
detection device; the means for accurately controlling the internal
magnetic field of the device; and the means for the accurate
measurement of the frequency or phase of the detected nuclear
moment precession signal of at least one of the nuclear moment
gases to provide a measurement of the angular rotation rate or
angular displacement, respectively, of the device with respect to
inertial space, thereby constituting an NMR gyroscope.
More particularly, a steady magnetic field is applied to an NMR
cell which is substantially shielded from other steady magnetic
fields. The NMR cell contains a gas or vapor of a substance that
possesses a magnetic moment that can be aligned by optical pumping
together with one or more additional gases, each of which possesses
a nuclear magnetic moment. The NMR cell is illuminated by optical
pumping light which has a directional component that is parallel to
the direction of the steady magnetic field and which has the proper
wavelength to be absorbed by the optically pumpable substance and
partially align the magnetic moments of that substance. The nuclear
moments of the nuclear moment gases are caused to become aligned
and are caused to precess at their respective Larmor precession
frequencies about the direction of the steady magnetic field. An AC
magnetic field at a suitable carrier frequency is also applied to
the NMR cell and the cell is illuminated by detection light which
has a directional component that is orthogonal to the direction of
the AC carrier magnetic field and which has a wavelength that is
essentially the same as that of the optical pumping light. The
intensity of that part of the detection light that is transmitted
by the cell is modulated in accordance with the totality of the
magnetic fields present in the cell, including the magnetic fields
that are generated by the precessing nuclear magnetic moments.
These modulations of the transmitted light intensity are detected
by a photodetector, after which they are electronically demodulated
to obtain signals at the Larmor precession frequencies of the
nuclear moment gases.
In one embodiment, the alignment of the nuclear magnetic moments of
each nuclear moment gas is accomplished by collisional interactions
between the atoms of the optically pumpable substance and the atoms
of the nuclear moment gas or gases. Sustained precession of the
nuclear magnetic moments of each nuclear moment gas is accomplished
by the application of an AC feedback magnetic field at the Larmor
precession frequency of the nuclear moment gas in a direction that
is orthogonal to the direction of the steady magnetic field. The AC
carrier magnetic field is applied at substantially the Larmor
precession frequency of the optically pumpable substance and in a
direction that is substantially parallel to the direction of the
steady magnetic field, thereby permitting the device to be operated
at higher values of the steady magnetic field strength and with
correspondingly higher Larmor precession frequencies for the
nuclear moment gases.
In the preferred embodiment, an optically pumpable substance such
as an alkali metal vapor is placed in an NMR cell together with two
noble gases and the nuclear magnetic moments of both noble gases
are aligned simultaneously by collisional interactions between the
atoms of the alkali metal atoms and the atoms of the two noble
gases. In this preferred embodiment of the invention, the alkali
metal is rubidium and the noble gases are krypton-83 and
xenon-129.
Another feature of the invention involves the use of at least one
buffer gas in substantial quantities in the NMR cell.
In accordance with still another feature of the invention, the
magnitude of the steady magnetic field is caused to remain constant
by feedback control of the field in such a way that the difference
between the Larmor precession frequencies of the two noble gases in
the NMR cell is caused to be equal to a predetermined constant
value.
In accordance with yet another feature of the invention, one of the
Larmor precession frequencies is compared to a precision reference
frequency and the resulting difference frequency is utilized to
provide a measurement of angular displacement or angular rate of
the device about the direction of the steady magnetic field.
These and other features of the invention will be made clear with
reference to the sections entitled "Principles of the Invention"
and "Detailed Description of the Preferred Embodiment."
It is the object of this invention to provide an NMR gyroscope
utilizing nuclear moment gases that have long relaxation times.
It is another object of this invention to provide a technique for
obtaining nuclear magnetic moment alignment and nuclear magnetic
resonance in these gases.
It is still another object of this invention to provide a technique
for detecting the Larmor precession frequencies of these gases.
It is yet another object of this invention to provide a technique
for detecting and controlling the internal magnetic field
environment of the gyroscope.
PRINCIPLES OF THE INVENTION
An NMR gyroscope operates on the principle of sensing angular
rotation rate as a shift in the Larmor precession frequency of one
or more nuclear species that possess nuclear magnetic moments.
Many atomic isotopes (usually those with odd atomic mass number)
possess an inherent angular momentum (spin) associated with the
nucleus. Always coexisting with such nuclear angular momentum is a
magnetic moment parallel with it. The ratio between the nuclear
magnetic moment and the nuclear angular momentum is a constant,
.gamma., called the gyromagnetic ratio, which has a particular
value for each type of isotope.
If a nuclear magnetic moment is placed in a magnetic field, with
any orientation other than being parallel to the direction of the
field, then the magnetic moment will precess about the direction of
the field with an angular frequency, .omega..sub.L, called the
Larmor precession frequency, which is equal to:
where .gamma. is the gyromagnetic ratio and H is the magnetic field
strength. Each isotope, therefore, has a characteristic Larmor
precession frequency in a given magnetic field.
If a system containing atoms that collectively have a precessing
magnetic moment is itself rotating at an angular rate,
.omega..sub.r, about the direction of H, then the observed
precession frequency will be shifted by an amount equal to that
rotation rate so that the observed Larmor precession frequency will
become:
Thus, a measurement of the observed Larmor frequency, .omega., can
be used as a measure of this rotation rate if both .gamma. and H
are known.
If the Larmor precession frequencies of two isotopes, each having a
different value of .gamma., are measured in the same magnetic
field, then the rotation rate can be measured without direct
knowledge of the value of the magnetic field. The equations for the
two isotopes are:
where .omega..sub.a and .omega..sub.b are the observed Larmor
frequencies of the two isotopes having gyromagnetic ratios
.gamma..sub.a and .gamma..sub.b, respectively. Solving these
equations for either H or .omega..sub.r gives the following
expressions:
which is independent of the angular rotation rate, .omega..sub.r,
and
which is independent of the magnetic field strength, H.
In one of the embodiments of this invention, the magnetic field
strength is caused to be constant by controlling the field in such
a way that the frequency difference, .omega..sub.a -.omega..sub.b,
between the two observed Larmor precession frequencies is always
equal to a constant. Specifically, two precision reference
frequencies, .omega..sub.a ' and .omega..sub.b ', which are derived
from a very stable, common frequency source, are chosen such that
.omega..sub.a ' is approximately equal to .gamma..sub.a H, and
.omega..sub.b ' is approximately equal to .gamma..sub.b H, and
their ratio accurately satisfies the following relationship:
The magnetic field strength is then servo-controlled in such a way
that the measured frequency difference between the two observed
Larmor precession frequencies is always caused to be equal to the
frequency difference between the two precision reference
frequencies, namely:
As a consequence of imposing the two conditions defined by
equations (6) and (7), it follows that the magnetic field strength
is equal to:
which is a constant, and that the angular rotation rate is equal
to:
and can therefore be readily obtained by measuring the difference
between either one or the other of the observed Larmor precession
frequencies and its associated precision reference frequency.
In addition to the basic phenomenon of magnetic moment precession
and the mathematical basis for the signal processing mechanization
which permits angular rotation rate information to be measured, as
described above, there are several other physical phenomena
involved in the implementation of a practical nuclear magnetic
resonance gyroscope. Those that will be described are the alignment
of nuclear magnetic moments, the achivement of sustained precession
of these moments, and the optical detection of the precessing
moments to provide a signal from which the Larmor precession
frequency can be determined.
The magnitude of an individual nuclear magnetic moment is extremely
small and the natural equilibrium condition is one in which a
nearly random orientation of moments exists in an ensemble of
atoms. Techniques must be used to orient a significant fraction of
these magnetic moments in a single direction so that a macroscopic
magnetic moment, and consequently a measureable signal, will be
produced.
The technique that is used for aligning nuclear magnetic moments,
as embodied in this invention, is a two-stage process and will be
referred to as "pumping." The two nuclear magnetic moment gases,
which are noble gases in the preferred embodiment of this
invention, are combined with an alkali metal vapor in a single,
optically transparent cell. This cell is illuminated by a
spectrally filtered, circularly polarized, beam of light which is
emitted by an alkali metal vapor electric discharge lamp. A steady
magnetic field is applied in such a direction that a significant
component of this field is parallel to the direction of the light
that is incident on the cell.
The first stage of pumping is an optical pumping process in which
the alkali metal vapor atoms are optically pumped by absorption of
some of the incident light. This results in the alignment of a
significant fraction of the atomic magnetic moments of the alkali
atoms in a direction that is parallel to that of the applied steady
magnetic field.
The second stage of pumping is a spin exchange pumping process in
which some of the alignment of the atomic magnetic moments of the
alkali atoms is transferred to the nuclear magnetic moments of the
noble gas atoms by spin exchange interactions during collisions
between the alkali atoms and noble gas atoms. This results in the
alignment of a significant fraction of the nuclear magnetic moments
of the noble gas atoms in a direction that is parallel to that of
the steady magnetic field. This spin exchange pumping technique is
an extension of the techniques of Bouchiat, Carver, and Varnum
(Phys. Review Letters 5, page 373, [1960]). In particular, as
embodied in this invention, it has been extended to include the
simultaneous alignment of the nuclear magnetic moments of two
different noble gas isotopes contained in the same cell.
The aligned magnetic moments of the alkali system and of both noble
gas systems of atoms are subject to relaxation mechanisms which
cause their alignments to decay exponentially with time towards
their natural equilibrium condition of random orientation. Each
system of moments is characterized by a relaxation time constant
which depends upon the kinds and quantities of all other
constituents and upon the total environment in the NMR cell. The
steady state fractional alignment of each system of moments is a
function of both the pumping rate and the relaxation time for that
system, with larger fractional alignments, hence larger signal
amplitudes, being achieved when the relaxation times are also long.
Accordingly, in order to achieve the longest possible relaxation
times, a suitable amount of a buffer gas such as helium or nitrogen
is also contained in the cell in order to reduce the relaxation
effects due to interactions of the magnetic moments with the walls
of the cell. Furthermore, particular isotopes of particular noble
gases have been chosen as the nuclear magnetic moment gases
specifically because of their long relaxation times.
Precession of the two systems of noble gas magnetic moments is
started and sustained by applying two AC magnetic fields in a
direction which is orthogonal to that of the applied steady
magnetic field. These fields have frequencies that are equal to the
respective Larmor precession frequencies of the two noble gases and
are referred to as the AC feedback magnetic fields inasmuch as they
provide the feedback signal function that is necessary in any
oscillator in order to achieve sustained oscillations. These
feedback fields cause each individual system of noble gas magnetic
moments to be torqued coherently away from the direction of their
initial alignment, which is parallel to that of the steady magnetic
field, into a plane which is orthogonal to the direction of the
steady magnetic field. The magnetic moments of each system precess
continuously in this plane, thereby creating two macroscopic
magnetic moments throughout the volume of the NMR cell, hence two
magnetic fields, which rotate in this plane at the respective
Larmor precession frequency of the two noble gases. The physics
associated with the torquing of spinning bodies requires that the
phases of the applied feedback fields be in quadrature with the
respective phases of the precessing nuclear magnetic moments.
These precessing nuclear magnetic moments are optically detected
using an approach which is adapted from a magnetometer technique
that was first developed in France by C. Cohen Tannoudji, J.
Dupont-Roc, S. Haroche, and F. Laloe (Rev. de Phys. Appl. 5, 95
[1970]). This magnetometer technique works on the principle that
the degree of absorption of optical pumping light by the alkali
atoms in the NMR cell depends upon the directional orientation of
the magnetic moments of the individual alkali atoms with respect to
the direction of the incident light. Both of the two rotating
magnetic fields, which are created by the two systems of precessing
noble gas nuclear magnetic moments, individually and simultaneously
exert torques on the precessing alkali magnetic moments, thereby
imparting nutational motions to the precessing alkali moments
which, in turn, modulate the intensity of the transmitted light.
The mathematical description and the salient characteristics of
this optical detection process can be summarized very briefly as
follows:
As embodied in this invention, a sinusoidal AC magnetic field,
H.sub.1 cos .omega..sub.c t, which will be referred to as the
carrier magnetic field, is applied to the cell and the direction of
this carrier magnetic field is used to define the z-axis. A steady
magnetic field is also applied to the cell essentially in the
direction of the z-axis. The components of all magnetic fields,
excluding the carrier magnetic field, are denoted as H.sub.x,
H.sub.y, and H.sub.z. The optical pumping light is incident on the
cell in the x-z plane and has components I.sub.x and I.sub.z, which
produce alkali magnetization components M.sub.x and M.sub.z.
It can be shown that under the following magnetic field
conditions:
where .gamma. is the gyromagnetic ratio for the alkali atom, .tau.
is the total alkali relaxation time under the influence of the
light absorption and relaxation process, .omega..sub.c is the
frequency of the carrier magnetic field, and n is an integer, that
the x-component of the transmitted light intensity, I.sub.tx, is
described (excluding a constant term) by the relation: ##EQU1##
where k is a constant and J.sub..+-. is defined as:
and where J.sub.n and J.sub.n.+-.p are Bessel functions of order n
and n.+-.p, respectively, with the same argument .gamma.H.sub.1
/.omega..sub.c, and H.sub.1 and .omega..sub.c are the amplitude and
frequency, respectively, of the carrier magnetic field.
We note several aspects of equation (11) which are relevant to this
invention:
(a) The x-component of the transmitted light intensity, I.sub.tx,
consists of a sum of harmonics of the carrier frequency,
.omega..sub.c.
(b) The in-phase (cos p.omega..sub.c t) response is linear in the
field H.sub.y for small values of H.sub.y.
(c) The quadrature-phase (sin p.omega..sub.c t) response is linear
in the field H.sub.x for small values of H.sub.x.
(d) The x-component of the transmitted light intensity can be made
to be linear in either H.sub.x alone or in H.sub.y alone by
choosing a particular amplitude for the carrier magnetic field,
H.sub.1, such that either J.sub.30 or J.sub.-, respectively, equals
zero. For n=1, it is most convenient to set H.sub.1 such that
J.sub.2 (.gamma.H.sub.1 /.omega..sub.c)=J.sub.o (.gamma.H.sub.1
/.omega..sub.c), whereby the p=1 term of J equals zero. The cos
.omega..sub.c t term of equation (11) can then be used to produce
usable H.sub.y information, and the sin 2.omega..sub.c t term can
be used to develop usable H.sub.x information.
(e) The response of the x-component of the transmitted light
intensity to either the H.sub.x or the H.sub.y magnetic field
components is proportional to the product of the x-component of the
incident light and the z-component of the magnetization, I.sub.x
M.sub.z. The incident light beam must therefore have components in
both the x-direction and the z-direction.
(f) Due to the conditions imposed on the magnetic fields, as
defined by equation (10) for the case n.noteq.0, the steady
magnetic field must be applied essentially in the z-direction and
the precession of the nuclear moments must occur essentially in the
x-y plane. In particular, these precessing moments create a
macroscopic magnetic field that rotates at the Larmor precession
frequency and which has an amplitude that is proportional to the
fractional alignment of the nuclear magnetic moments. This rotating
magnetic field is responsible for a term in the x-component of the
transmitted light intensity that is due to the y-axis component of
this field, namely:
where h.sub.a is the amplitude of this rotating magnetic field and
.omega..sub.a is the Larmor precession frequency of the nuclear
moment gas. It is this term which is utilized for extracting the
nuclear Larmor precession frequencies in the embodiment of this
invention. The preceding analysis is valid for steady magnetic
fields and also for slowly varying fields, including the above
rotating magnetic field, in particular, provided that the condition
.omega..sub.a .tau.<1 is satisfied.
(g) The effects of steady magnetic field components of H.sub.x and
H.sub.y can be separately determined from the light intensity
modulations and this makes it possible to independently measure or
control these field components.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional drawing showing the physical arrangement of
components of an NMR gyro sensor assembly.
FIG. 2A is a perspective drawing illustrating a portion of the
magnetic coils for generating the z axis field.
FIG. 2B is a perspective drawing illustrating the portion of the
magnetic field coils for generating the x axis and y axis
fields.
FIG. 3 is a conceptual diagram illustrating the processes of
optical pumping and of modulation of the intensity of the light
that is transmitted by the NMR cell.
FIG. 4 is a block diagram indicating the functional mechanization
of the electronics of an NMR gyro.
FIG. 5 is a conceptual diagram showing an alternative configuration
of an NMR gyro sensor assembly. FIG. 5 also serves to illustrate
the configuration of a research apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIG. 1, which is a sectional drawing showing the
physical arrangement of components of an NMR gyro assembly, a
rubidium vapor lamp 10, which is excited by a high frequency power
source, is used to emit light contaning the spectral lines of
rubidium. This lamp is similar in design to that described by Bell,
Bloom and Lynch (Rev. Sci. Instr. 32, 688 [1961]). The lamp 10 is
housed in an enclosure 12 which is used to maintain the lamp at an
elevated temperature suitable for maximum light emission. The light
passes through a glass condenser lens 14 and through a plastic
fresnel collimating lens 16 before passing through an optical
interference filter 18. This filter is designed to transmit most of
the 794.7 nanometer wavelength light from one spectral line of the
rubidium while blocking most of the 780.0 nanometer wavelength
light from an adjacent spectral line. The filtered light passes
through a second fresnel collimating lens 20, is reflected in a
prism 22 to change its direction and converges on the end of an
input fiber optics bundle 24. This fiber optics bundle then
transmits the light towards the center of the device and makes a
bend so that the light leaves the end 25 of the bundle 24 with a
mean angle of about 45 degrees relative to the vertical as shown in
the drawing. The vertical axis as shown in the drawing is
designated as the z-axis. The x-axis is defined to be pointing to
the left in the drawing. Thus, FIG. 1 is a sectional drawing in the
x-z plane. The light leaving the bundle passes through a circular
polarizer 26, and enters the NMR cell 28.
The NMR cell 28 is a sealed, optically transparent, glass
cylindrical enclosure containing a small quantity of isotopically
enriched rubidium-87 metal, approximately 0.5 torr of isotopically
enriched xenon-129 gas, approximately 20 torr of isotopically
enriched krypton-83 gas, and a buffer gas consisting of either
about 400 torr of helium-4 or about 100 torr of nitrogen. These are
introduced into the cell in the order stated while the cell is
attached to a vacuum filling station and the cell is then sealed
off.
The cell 28 is mounted in a temperature controlled alumina oven 30
which is heated and controlled by a resistance band heater 32 that
uses a high frequency power source. The oven is maintained at a
temperature of about 65.degree. C., at which temperature
approximately one-half of the light entering the cell 28 is
absorbed. Most of the light that is not absorbed in the cell 28
enters an output fiber optics bundle 36 and passes through a lens
38 to a silicon photodetector 40. Other components shown in this
drawing are a magnetic field coil structure 34, which will be
described in more detail below, (FIG. 2) a set of several layers of
magnetic shielding 42 designed to attenuate the influence of
external magnetic fields, and a supporting structure 44.
The magnetic field coil structure 34 consists of a machinable glass
(Corning "MACOR") cylindrical coil form, on the outer surface of
which grooves have been cut and then wires laid in the grooves to
form the magnetic field coils.
FIGS. 2A and 2B are a perspective drawing illustrating the
configuration of the magnetic field coils that are a part of FIG. 1
(See U.S. Pat. No. 4,063,207) FIG. 2A shows the coil form 34' and
the main solenoidal coil windings 50 that produce a magnetic field
that is parallel to the axis of the cylinder, which is designated
as the z-axis. Additional coil windings 52 at the ends of the coil
form are employed to improve the spatial uniformity of the magnetic
field. The coil windings 52 are commingled with the coil windings
50. The combination of 50 and 52 will be referred to as the z-axis
field coils.
FIG. 2B shows the same coil form 34' and two additional coil pairs
that provide magnetic fields along two axes that are mutually
orthogonal to each other and to the axis of the cylinder. Coil pair
54 provides a magnetic field along the x-axis and coil pair 56,
only one member of which is visible in the drawing, provides a
magnetic field along the y-axis.
FIG. 3 is a conceptual diagram illustrating for each of the noble
gases the processes of optical pumping and of modulation of the
intensity of the light that is transmitted through the NMR cell.
Because these processes are so similar for the two noble gases,
they are illustrated and described for only one of the two noble
gases. In particular, they apply for the case n=1, where n is as
used in equations (11) and (12). The circularly polarized light
which enters the NMR cell 28' has a component 64 along the z-axis,
which is referred to as optical pumping light, and a component 66
along the x-axis, which is referred to as detection light. Through
the interactions of the optical pumping light 64 and the steady
magnetic field 68, the rubidium atoms 60 have their magnetic
moments aligned preferentially in the z-direction. By interatomic
collisions, this magnetic moment alignment is transferred from the
rubidium atoms 60 to the noble gas nuclei 62.
A sinusoidal AC feedback magnetic field 70 that is matched in
frequency and phase to the Larmor precession frequency of the
collective magnetic moment of the noble gas nuclei 62 is applied in
the x-direction and serves to torque the magnetic moment of these
nuclei to the x-y plane. This component of noble gas nuclear
magnetic moment then precesses in the x-y plane at the noble gas
Larmor precession frequency, .omega..sub.a, about the steady
magnetic field 68. This precessing nuclear magnetic moment
component create a nuclear precession magnetic field of strength,
h.sub.a, that rotates in the x-y plane and which therefore has a
component in the y-direction that is equal to (h.sub.a cos
.omega..sub.a t).
The detection light 66 interacts with the rubidium atoms 60, which
are under the influence of the steady magnetic field 68, a
superimposed AC carrier magnetic field 69, and the y-component of
the nuclear precession field, h.sub.a. This interaction causes the
intensity of the x-component of the transmitted light 72 to be
modulated at the carrier frequency, .omega..sub.c, with a
modulation envelope 74 at the nuclear precession frequency,
.omega..sub.a. These light modulations are then converted into
electrical signals by the silicon photodetector 40'.
With reference to FIG. 4, which is a block diagram indicating the
functional mechanization of the electronics of an NMR gyro, light
from the light source 10 enters the device through the input optics
82 and passes through the NMR cell 28. Input optics 82 comprises
items 14 through 26 as discussed above. The light that is not
absorbed and which is modulated in intensity, as described above
(FIG. 3), is transmitted by means of the output optics 86 to the
photodetector 40 where the light intensity modulations are
converted into an electrical signal 89. Output optics 86 comprises
items 36 and 38 as discussed above. The signal 89 is first
amplified and then synchronously demodulated in two separate
conditions in a carrier signal detector 90 in order to generate
control signals for the x-axis and y-axis magnetic fields.
A DC signal 93 for controlling the y-axis DC magnetic field is
generated by synchronously demodulating the signal 89 using a
sinusoidal reference signal having a frequency, f.sub.c ', that is
derived from a crystal controlled precision reference frequency
source 92. The frequency and phase of the sinusoidal signal from
source 92 are the same as those of the applied AC carrier magnetic
field. The amplitude of the DC control signal 93 is proportional to
the amplitude of that component of the light intensity modulations
at the carrier frequency that is in phase with the applied AC
carrier magnetic field. By reference to equation (11), this DC
signal 93 is also proportional to the value of the y-axis magnetic
field. The DC control signal 93 is summed at point 95 with an
additional constant DC signals 94 that is generated in the DC power
supplies 96 and the used to is used supply the total DC current to
the y-axis magnetic field coil 56. The y-axis DC magnetic field is
thereby controlled in such a way that the amplitude of the DC
signal 93 remains close to zero, which results in a suppressed
carrier mode of operation. In this manner, changes in the y-axis
magnetic field are sensed and corrected to maintain carrier
suppression.
In a similar manner, a DC signal 104 for controlling the DC
component of the x-axis magnetic field is generated by
synchronously demodulating the signal 89 usng a sinusoidal
reference signal having a frequency, 2f.sub.c ', that is derived
from a crystal controlled precision reference frequency source 102.
By setting J.sub.o (.gamma.H.sub.1 /.omega..sub.c)=J.sub.2
-(.gamma.H.sub.1 /.omega..sub.c), for n=p=1, the coefficient of the
sin .omega..sub.c t term becomes zero, and a higher harmonic is
then used. The reference signal at 2f.sub.c ' generated by source
102 is timed relative to the reference signal f.sub.c ' such that
when the f.sub.c ' signal is represented as cos .omega..sub.c 't,
the 2f.sub.c ' signal is represented as sin 2 .omega..sub.c 't. The
amplitude of the DC control signal 104 is proportional to the value
of the x-axis magnetic field. The DC control signal 104 is summed
at point 107 with an additional constant DC signal 106 that is
generated in the DC power supplies 96 and the resultant is used to
supply the total DC current to the x-axis magnetic field coil 54.
In this manner, the value of the DC component of the x-axis
magnetic field is controlled to be essentially equal to zero.
In addition to the DC signal 93 resulting from the synchronous
demodulation at the frequency, f.sub.c ' in the carrier signal
detector 90 there are AC signals 109 which are proportional to the
AC components of the y-axis magnetic field. Of particular interest,
are the modulations at the nuclear Larmor precission frequencies.
These signals are separated and filtered in a nuclear precession
signal separator 110 to yield a signal 112 at the xenon-129
precession frequency, f.sub.a, of about 135 hertz, a signal 114 at
the krypton-83 precession frequency, f.sub.b, of about 19 hertz,
and a signal 116 at their difference frequency, f.sub.a -f.sub.b,
of about 116 hertz. These stated values for the nuclear Larmor
precession frequencies are for a z-axis steady magnetic field value
of 0.114 gauss which is used in the preferred embodiment.
A DC signal 122 for controlling the DC component of the z-axis
magnetic field is generated by comparing the precession difference
frequency, f.sub.a -f.sub.b, 116 in a frequency comparator 118 to a
reference frequency, f.sub.a '-f.sub.b ', that is generated by the
crystal controlled precision reference frequency source 120. Any
phase difference between the signals 116 and 120 creates a DC
control signal 122 which is summed at point 123 with an additional
constant DC signal 126 that is generated in the DC power supplies
96 and the resultant signal 125 is used to supply the total DC
current to the z-axis magnetic field coil 124 which comprises coils
50 and 52. In this manner, the value of the DC component of the
z-axis magnetic field is controlled to be equal to a specific
constant value as given by equation (8).
A sinusoidal AC current 128, that is generated by the carrier field
supply 130, is also applied to the z-axis magnetic field coil 124
to produce an AC carrier magnetic field. The AC carrier current 128
is summed at point 127 with the DC currents 125 and the resultant
comprises the total current supplied to the z-axis magnetic field
coil 124. The sinusoidal AC carrier current 128 has a frequency,
f.sub.c ', that is generated by the crystal controlled precision
reference frequency source 92, which is the same as the signal used
as a reference signal for the carrier signal detector 90. The
carrier frequency, f.sub.c ', is about 80,000 hertz, which is equal
to the Larmor precession frequency of rubidium-87 for a z-axis
steady magnetic field value of 0.114 gauss which is used in the
preferred embodiment.
The amplitude of the AC carrier current 128 is selected to have a
specific value such that the amplitude of the sinusoidal AC carrier
magnetic field is equal to a particular factor times the DC
component of the z-axis magnetic field which is produced by the DC
current 125. In the preferred embodiment, this factor has a value
of 1.84 and the amplitude of the AC carrier magnetic field is made
equal to 0.210 gauss. In this manner, the amplitude of the
component of the signal 89 at the carrier frequency, f.sub.c ', is
made to be insensitive to x-axis magnetic fields. The mathematical
basis for this preferred relationship between the two fields is
contained in equations (11) and (12) for the case n-1 and p=1.
Two feedback magnetic fields are created along the x-axis in order
to achieve sustained precession of the nuclear magnetic moments of
xenon-129 and kyrpton-83. The xenon-129 signal 112 is used in an AC
feedback magnetic field generator 144 to generate a sinusoidal AC
feedback signal 148 which has a constant amplitude and a frequency
and phase that are identical with those of the xenon-129 signal
112. The signal 148 is summed with a similarly generated sinusoidal
AC feedback signal 146 that is derived from the krypton-83 signal
114. The sum 150 of the two AC feedback currents 146 and 148 is
further summed at point 107 with the DC currents 104 and 106 and
the resultant comprises the total current that is supplied to the
x-axis magnetic field coil 54. The function of the AC feedback
magnetic fields is to continuously torque the xenon and krypton
nuclear magnetic moments, that have been newly aligned along the
z-axis, into the x-y precession plane to replenish those moments
that have been lost through nuclear magnetic moment relaxation
processes. In this manner, the sustained precession of xenon and
krypton magnetic moments creates two steady state magnetic fields
that rotate in the x-y plane and which consequently produce steady
state light intensity modulations at the Larmor precession
frequencies, f.sub.a and f.sub.b.
The angular rotation rate of the gyro is obtained by comparing the
Larmor precession frequency, f.sub.a, of the xenon-129 signal 112
in a frequency comparator 134 with a reference frequency f.sub.a ',
that is derived from a crystal controlled precision reference
frequency source 136. The resultant difference frequency f.sub.a
'-f.sub.a, is equal to the angular rotation frequency, f.sub.r, of
the gyro, in accordance with equation (9), and this data 138 is
sent to a computer for further processing. The gyro angular
rotation rate data 138 contains both frequency information and
phase information and therefore contains both angular rate
information and angular displacement information, respectively.
All precision reference frequency sources 92, 102, 120 and 136 are
driven by a common crystal controlled master oscillator 152 by
digital multiplication and division techniques. The frequency of
the master oscillator 152 is denoted as f.sub.m ' in FIG. 4. The
angular rotation rate data 138 is, to first order, independent of
the frequency stability of the master oscillator 152.
With reference to FIG. 5, which is a conceptual diagram showing an
alternative embodiment of an NMR gyro sensor assembly, items
identified with primed numerals are functionally similar to the
corresponding unprimed items. Rubidium lamp 10' supplies optical
pumping light through the input light pipe 24' to the NMR cell 28'.
The lamp 10' also supplies detection light to the NMR cell 28'
through a second channel which includes input light pipe 154 and
input prism 155. The detection light that is transmitted by the NMR
cell 28' passes through the output prism 158 and output light pipes
156 and 160 to the photodetector 40'. Suitable magnetic fields are
applied to the NMR cell through the three-axis Helmoltz coil
assembly 161, 162 and 163, which in this arrangement are the field
coils for the z-axis, y-axis, and x-axis, respectively. The
direction of the input light through the light pipe 24' is here
defined to be along the z-axis, the x-axis is up in the drawing,
and the y-axis is out of the paper.
The arrangement shown in FIG. 5 is an alternative to that of FIG. 1
which serves to emphasize that optical detection should be
accomplished in a direction that is transverse to that of the
steady magnetic field which is along the z-axis. This may be
accomplished either as shown in FIG. 1 using a 45 degree or other
similar angle between the direction of the light beam through the
NMR cell relative to the direction of the steady magnetic field, or
as shown in FIG. 5 using two separate light paths with the pumping
light being parallel with the direction of the steady magnetic
field and the detection light being transverse to the direction of
the field. This arrangement also includes the possibility that the
pumping and detection light beams could originate from separate
light sources and that they could also have different spectral or
polarization characteristics.
With certain modifications, FIG. 5 can also be used to illustrate
the configuration of a research apparatus that is especially useful
for performing experimental investigations of the properties of
noble gas-alkali vapor systems. The modifications consist of
deleting the detection light path 154, 155, 156. 158, 160 and 40',
and adding the output light path 174 and 175. For this application,
which corresponds to the case of n=0 as used in equations (11) and
(12), the coordinate axes are relabeled, with the x-axis and z-axis
being interchanged from before, so that the direction of the input
light through the light pipe 24' is redefined as being along the
x-axis and the z-axis is up in the drawing. The input light passes
through the cell 28' and into an output light pipe 174 which
transmits the light to the photodetector 175. The AC carrier
magnetic field is applied using the z-axis field coil 163 and a
small DC field of approximately 100 microgauss is applied using the
y-axis field coil 162. During operation, a larger DC field of
approximately 10 milligauss is applied by the x-axis field coil 161
during the initial nuclear magnetic moment spin exchange pumping
time. At the conclusion of the pumping time, which is typically a
few minutes, this field is quickly turned off leaving the aligned
nuclear magnetic moments to precess in the x-z plane, which is in
the plane of the paper. The z-axis component of the precessing
nuclear magnetic field produces light intensity modulations
analogous to the ones described above. This mode of operation is
quite similar to that described by Cohen-Tannoudji, et al. (ibid)
except that in this alternative embodiment the rubidium magnetic
moments that are used for detection and the noble gas nuclear
magnetic moments that are used for nuclear Larmor precession are
located in the same cell 29'. The close association of the rubidium
atoms during collisions with the noble gas atoms causes the
rubidium atoms to sense a much larger average magnetic field from
the noble gas nuclei. This proximity effect results in signals that
are much larger than might otherwise be detectable. This apparatus
is therefore especially useful for research studies on the
properties of the noble gas-alkali vapor system.
RELATED PATENTS
A number of patents which relate to the fields of this invention
are set forth below. A review of these references indicates that
none of them discloses the novel features set forth in the claims
of the present specification. However, it is considered appropriate
for the benefit of the Patent Office to include all prior art that
was discovered during the searches and these references are
therefore listed below:
__________________________________________________________________________
Patent No. Inventor Title Issued
__________________________________________________________________________
3,103,623 I. A. Greenwood, Jr. Nuclear Gyroscope 9-10-63 3,103,624
I. A. Greenwood, Jr., Nuclear Gyroscope 9-10-63 et al. 3,396,329 A.
Salvi Magnetic Resonance Mag- 8-6-68 netometers for Measuring Weak
Magnetic Fields From Aboard a Moving Vehicle as a Plane 3,404,332
A. Abragam, et al. Magnetic Resonance De- 10-1-68 vices for
Accurately Measuring Magnetic Fields in Particular Low Magnetic
Fields, on Board of a Movable Body 3,500,176 A. Kastler, et al.
Method and Apparatus 3-10-70 for Controlling a Mag- netic Field
Employing Optically Pumped Nuclear Resonance 3,513,381 W. Happer,
Jr. Off-Resonant Light as a 5-19-70 Probe of Optically Pumped
Alkali Vapors 3,729,674 J. R. Lowdenslager Digital Nuclear Gyro-
4-24-73 scopic Instrumentation and Digital Phase Locked Loop
Therefore
__________________________________________________________________________
In conclusion, the present invention has been described in terms of
particular elements and particular physical arrangements, but it is
clear that reasonable alternatives, such as the use of different
optical paths accomplishing the same results, or the use of
different combinations of the noble gases or the use of a different
substance than rubidium, or the use of other values for the
frequencies or magnetic fields mentioned in the foregoing
specification, may all be within the scope of the present
invention.
* * * * *